1. Field of Invention
The invention relates to a technical field of a hydraulic control circuit for working machines, such as hydraulic shovels.
2. Description of Related Art
In some working machines, such as hydraulic shovels, an offset-type working implement capable of swinging to the left and right is attached to a machine body. Generally, such working machines accompany a risk that when the working implement is moved, it may contact, or interfere, with an operator's seat portion. In such working machines, therefore, consideration must be paid to avoiding contact between the working implement and the operator's seat portion.
To that end, there has hitherto been proposed a working machine having attitude detecting means for detecting an attitude of a working implement, and a control unit for determining based on a detection signal from the attitude detecting means whether the working implement comes within a predetermined zone around the operator's seat portion. When it is determined that the working implement comes within the predetermined zone around the operator's seat portion, the control unit outputs a control command to a hydraulic circuit, for a hydraulic actuator associated with the working implement, so that the working implement is stopped.
One known example of such a related art device is shown in FIG. 9. In the example, solenoid proportional pressure reducing valves 49A, 49B, which operate in accordance with a command from a control unit, are provided between a pilot-operated control valve 46 for controlling the supply of pressurized oil to a hydraulic actuator 45, e.g., a boom cylinder, and pilot valves 48A, 48B for delivering pilot pressurized oil upon manipulation of a control member 47. When the working implement is positioned away from the operator's seat portion, the solenoid proportional pressure reducing valves 49A, 49B are opened to allow the supply of the pilot pressurized oil to the control valve 46. On the contrary, when the working implement comes close to the operator's seat portion, the solenoid proportional pressure reducing valves 49A, 49B are closed to cut off the supply of the pilot pressurized oil to the control valve 46, thereby stopping the working implement.
The above prior art however has a problem. Because the solenoid proportional pressure reducing valves are closed to stop the working implement when the working implement comes close to the operator's seat portion, as described above, the working implement is stopped during the operation and the working efficiency is reduced. The invention intends to overcome such a problem.